Seafloor Spreading

www.ck12.org Chapter 1. Seaﬂoor Spreading

CHAPTER 1 Seaﬂoor Spreading

Lesson Objectives

• Describe the main features of the seafloor. • Explain what seafloor magnetism tells scientists about the seafloor. • Describe the process of seafloor spreading.

Vocabulary

• abyssal plains • echo sounder • seaﬂoor spreading • trench

Introduction

World War II gave scientists the tools to ﬁnd the mechanism for continental drift that had eluded Wegener. Maps and other data gathered during the war allowed scientists to develop the seaﬂoor spreading hypothesis. This hypothesis traces oceanic crust from its origin at a mid-ocean ridge to its destruction at a deep sea trench and is the mechanism for continental drift.

Seaﬂoor Bathymetry

During World War II, battleships and submarines carried echo sounders to locate enemy submarines ( Figure 1.1). Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. By knowing the speed of sound in seawater, scientists calculate the distance to the object based on the time it takes for the wave to make a round-trip. During the war, most of the sound waves ricocheted off the ocean bottom. This animation shows how sound waves are used to create pictures of the sea floor and ocean crust: http://earthguid e.ucsd.edu/eoc/teachers/t_tectonics/p_sonar.html . After the war, scientists pieced together the ocean depths to produce bathymetric maps, which reveal the features of the ocean floor as if the water were taken away. Even scientist were amazed that the seafloor was not completely flat ( Figure 1.2). The major features of the ocean basins and their colors on the map in Figure 1.2 include:

• mid-ocean ridges: rise up high above the deep seaﬂoor as a long chain of mountains; e.g. the light blue gash in middle of Atlantic Ocean. • deep sea trenches: found at the edges of continents or in the sea near chains of active volcanoes; e.g. the very deepest blue, off of western South America. • abyssal plains: ﬂat areas, although many are dotted with volcanic mountains; e.g. consistent blue off of southeastern South America.

When they ﬁrst observed these bathymetric maps, scientists wondered what had formed these features.

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FIGURE 1.1 This echo sounder has many beams and creates a three dimensional map of the seaﬂoor. Early echo sounders had a single beam and created a line of depth measurements.

FIGURE 1.2 A modern map of the southeastern Paciﬁc and Atlantic Oceans.

Seaﬂoor Magnetism

Sometimes – no one really knows why – the magnetic poles switch positions. North becomes south and south becomes north.

• Normal polarity: north and south poles are aligned as they are now. • Reversed polarity: north and south poles are in the opposite position.

During WWII, magnetometers attached to ships to search for submarines located an astonishing feature: the normal and reversed magnetic polarity of seaﬂoor basalts creates a pattern.

• Stripes of normal polarity and reversed polarity alternate across the ocean bottom. • Stripes form mirror images on either side of the mid-ocean ridges ( Figure 1.3). • Stripes end abruptly at the edges of continents, sometimes at a deep sea trench ( Figure 1.4).

The characteristics of the rocks and sediments change with distance from the ridge axis as seen in the Table 1.1.

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FIGURE 1.3 Magnetic polarity is normal at the ridge crest but reversed in symmetrical patterns away from the ridge center. This normal and reversed pattern continues across the seaﬂoor.

FIGURE 1.4 Seaﬂoor is youngest at the mid-ocean ridges and becomes progressively older with distance from the ridge.

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TABLE 1.1: Characteristics of crustal rocks with distance from ridge axis

Rock ages Sediment thickness Crust thickness Heat ﬂow At ridge axis youngest none thinnest hottest With distance from becomes older becomes thicker becomes thicker becomes cooler axis

A map of sediment thickness is found here: http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_sedimentthickn ess.html . The oldest seaﬂoor is near the edges of continents or deep sea trenches and is less than 180 million years old ( Figure 1.4). Since the oldest ocean crust is so much younger than the oldest continental crust, scientists realized that seaﬂoor was being destroyed in a relatively short time. This 65 minute video explains “The Role of Paleomagnetism in the Evolution of Plate Tectonic Theory”: http://o nline.wr.usgs.gov/calendar/2004/jul04.html .

The Seaﬂoor Spreading Hypothesis

Scientists brought these observations together in the early 1960s to create the seaﬂoor spreading hypothesis. In this hypothesis, hot buoyant mantle rises up a mid-ocean ridge, causing the ridge to rise upward ( Figure 1.5).

FIGURE 1.5 Magma at the mid-ocean ridge creates new seaﬂoor.

The hot magma at the ridge erupts as lava that forms new seafloor. When the lava cools, the magnetite crystals take on the current magnetic polarity. As more lava erupts, it pushes the seafloor horizontally away from ridge axis. These animations show the creation of magnetic stripes of normal and reversed polarity at a mid-ocean ridge: http://w ww.nature.nps.gov/GEOLOGY/usgsnps/animate/A49.gif ; http://www.nature.nps.gov/GEOLOGY/usgsnps/animat e/A55.gif . The magnetic stripes continue across the seafloor.

• As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis.

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• If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle. • The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust.

Seafloor spreading is the mechanism for Wegener’s drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that over millions of years takes them around the planet’s surface. The breakup of Pangaea by seafloor spreading is seen in this animation: http://www.scotese.com/sfsanim.htm . The breakup of Pangaea with a focus on the North Atlantic: http://www.scotese.com/natlanim.htm . An animation of the breakup of Pangaea focused on the Pacific Ocean: http://www.scotese.com/pacifanim.htm . Seafloor spreading is the topic of this Discovery Education video: http://video.yahoo.com/watch/1595570/5390151 . The history of the seafloor spreading hypothesis and the evidence that was collected to develop it are the subject of this video (3a): http://www.youtube.com/watch?v=6CsTTmvX6mc&feature=rec-LGOUT-exp_fresh+div-1r-2 (8:05).

MEDIA Click image to the left or use the URL below. URL: http://www.ck12.org/ﬂx/render/embeddedobject/5794

Lesson Summary

• Using technologies developed to fight World War II, scientists were able to gather data that allowed them to recognize seafloor spreading as the mechanism for Wegener’s drifting continents. • Bathymetric maps revealed high mountain ranges and deep trenches in the seafloor. • Magnetic polarity stripes give clues to seafloor ages and the importance of mid-ocean ridges in the creation of oceanic crust. • Seafloor spreading processes create new oceanic crust at mid-ocean ridges and destroy older crust at deep sea trenches.

Review Questions

1. Describe how sound waves are used to develop a map of the features of the seafloor. 2. Why is the oldest seafloor less than 180 million years when the oldest continental crust is about 4 billion years old? 3. Describe the major features and the relative ages of mid-ocean ridges, deep sea trenches, and abyssal plains. 4. Describe how continents move across the ocean basins as if they are on a conveyor belt. 5. If you were a paleontologist who studies fossils of very ancient life forms, where would be the best place to look for very old fossils: on land or in the oceans? 6. Imagine that Earth’s magnetic field was fixed in place and the polarity didn’t reverse. What effect would this have on our observations of seafloor basalts? 7. Look at a map of the Atlantic seafloor with magnetic polarity stripes and recreate the history of the Atlantic Ocean basin.

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